The present invention relates generally to coded signal demodulators, and more particularly to a minimum shift-keyed and offset-keyed quadrature-phase-shift-keyed signal demodulator that may be advantageously utilized in data communication systems.
A type of frequency-shift-keying commonly referred to as minimum-shift-keying (MSK) is particularly well suited for use in radio communications systems since the spectral energy is more easily contained within the limited bandwidth available than other binary frequency or phase-shift-keying modulations. An MSK signal is a continuous-phase-frequency-shift-keying (CPFSK) between one signalling tone and another signalling tone depending upon the value of a binary digit or bit. For example, a mark or binary one may be characterized by modulation with a 1200 Hz signalling tone, and a space or binary zero may be characterized by modulation with an 1800 Hz signalling tone. The continuous phase in CPFSK indicates that the phase-frequency changes between the two signalling tones are continuous; i.e., a signalling tone begins at the same phase as that which the previous signalling tone ended.
Also, to be MSK, the deviation ratio of the CPFSK must be equal to one-half. The deviation ratio is the frequency difference between the two signalling tones divided by the rate of transmission of the bits expressed in bits per second. In the above example of CPFSK with signalling tones of 1200 Hz, the transmission speed would be 1200 bits per second to be MSK.
Since MSK is a member of the frequency-shift-keying (FSK) family, the bits can be recovered from the MSK modulated signal by using any FSK signal detector or demodulator. In binary FSK one of two signalling tones are transmitted to indicate one of two possible values for a binary digit or bit. Any device which discriminates between the two signalling tones can be employed as a signal detector, such as a frequency discriminator, the error voltage from a phase lock loop, or a delay differential detector. All three methods are in common use as MSK signal detectors.
Better signal detectors can be constructed for MSK which advantageously exploit its unique characteristics; i.e., continuous phase and deviation ratio equal to one-half. These two properties result in phase characteristics usually found in a certain type of phase-shift-keying (PSK) modulation called offset-keyed quadrature-phase-shift-keying (OK-QPSK). Although OK-QPSK modulation encodes the data bits as phase shifts of the signal at bit boundaries relative to a reference carrier rather than a frequency of the signalling tone as in FSK, a relationship between the two methods exists. Because of this characteristic, coherent OK-QPSK signal detectors are also commonly used as signal detectors for MSK. Coherent signal detectors achieve more reliable signal detection than the three non-coherent detectors described earlier. This is because coherent detectors use knowledge of the exact phase of the signalling tones to detect the transmitted bits. As such, this type of demodulator usually consists of two parts; a device which extracts the bit clock in thephase of the signalling tones, and a detector which uses the extracted signal phase. A detector which uses knowledge of the signal phase is commonly implemented by a device called a matched filter.
All of these MSK demodulators can be built with varying amounts of hardware. The more reliable coherent demodulators usually require much more hardware than the simpler non-coherent demodulators. Some of the demodulators can be implemented using digital or discrete-time techniques. However, the faster the transmission rate or the more complex the demodulator, the higher the cost of implementation. As a result, a coherent demodulator, capable of accommodating data transmission speeds over several hundred bits per second and performing other supervisory functions (as is commonly required in a mobile radio), cannot be effectively implemented using a single low-cost microcomputer with existing techniques.